1. Field of the Invention
The present invention is directed to a wall detector and an associated measurement method for detecting objects such as iron reinforcements and plastic conduit which are enclosed in walls.
In the constructional industry, devices based on a radar technology (e.g., DE19847688) or capacitive UWB (ultra-wideband) technology (e.g., EP1478949) are used for locating objects in substrates, e.g., iron reinforcements or plastic conduit in walls (side walls, ceilings, floors). For this purpose, high-frequency alternating electric fields are introduced by a detection antenna into the substrate to be examined. The interaction of these high-frequency alternating electric fields with the objects is fed back with a delay to the detection antenna. There are devices of the kind mentioned above with measurement systems operating in the frequency domain and those that operate instead in the time domain. These known measurement systems for detection are not in themselves the object of the invention but form a necessary part of it.
2. Description of the Prior Art
Common for all of these measurement systems, is the use of a large frequency bandwidth. This bandwidth is necessary for achieving a sufficient depth resolution. Measurement frequencies in the band range of 0.1 GHz to 10 GHz are suitable for finding locations of objects in the constructional industry. This large band region is selected because, depending upon the application, a sufficient lateral resolution, a sufficient depth resolution, and a sufficient penetration (measurement depth) can be achieved in this way. Therefore, the choice of the center frequency of the frequency band is a compromise between resolution and measurement depth. The higher the frequency, the higher the resolution, but the higher also the attenuation and the lower the measurement depth that can be achieved.
The propagation velocity υ of the electromagnetic interaction in media is reduced relative to that of the speed of light in vacuum c0 by the divisor √{square root over (∈r)}, where ∈r is the relative permittivity of the medium. Therefore, in order to correctly measure the depth of objects in substrates by the time behavior or frequency behavior of the electromagnetic interaction, the relative permittivity of the substrate must be known. Even when the substrate (e.g., concrete) is always the same, the permittivity can change drastically as a function of the natural humidity. Dry concrete has a relative permittivity of roughly ∈concrete=5, whereas water has a static relative permittivity of ∈water=80. Because of the high porosity of concrete (and of most building materials such as brick, lightweight concrete, etc.) of about 14% by volume, concrete can also absorb much water. Humidity is therefore the dominant influencing factor for determining the relative permittivity ∈r of the substrate.
EP1478949 discloses a wall detector for detecting an object embedded in a substrate which has a detection antenna for introducing high-frequency alternating electric fields in the substrate, a measurement system connected to the detection antenna for measuring the delayed interaction of the alternating fields with the object, and a calculation device for detecting the object from the measurement signal and for determining the associated depth information. Separation of object signal components and substrate signal components is effected in a method step, with the relative permittivity of the medium being determined by correlation with reference values and begin used, together with the phase information of the object signal component, to determine the depth of the object.
Another factor influencing electromagnetic interaction is the salt content of the substrate. The salts are usually dissolved in interstitial water which leads to conductivity σ and, therefore, resistive losses, so that the absorption α by the substrate of the introduced electromagnetic interaction depends upon the penetration depth. The higher the absorption α by the substrate, the more power must be introduced into the substrate for a required signal-noise ratio to achieve the necessary dynamic for detecting the deep-lying objects.
According to DE19915016, a radar wall detector evaluates, in addition to the measurement signal, a high-frequency crosstalk signal proceeding from a transmission antenna and acting on a reception antenna at a distance laterally therefrom, for determining the absorption and permittivity of the substrate and regulates the output of the wall detector depending on the determined absorption.